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2007 Forester Sports XT 4EAT
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@Jfunk that last CAT isn't monitored, assumed it was installed to meet the stricter emissions some States have. I always thought moving the rear O2 sensor after the last CAT would reduce some of these P0420 CEL issues?

► Disclaimer - NOT recommendation to do this, just thinking out loud.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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2005 Forester X
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57 Posts
@2.5x_sleeper That's certainly an interesting idea. I was thinking about modifying my spacer a little bit to see if that does anything. Just drilling the hole out bit size by bit size. Right now the hole is just under a quarter inch big, so I wonder if I get slightly more flow in there, I could find the happy medium of no codes.
 

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2007 Forester Sports XT 4EAT
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@Jfunk that's certainly worth trying! The spark plug non-fouler hole is the same, just a touch smaller than a 1/4".

Some try the spark plug non-fouler with just one drilled spacer, moving to the second undrilled spacer if needed.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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I actually just looked closer at my spacer, and there is a snap ring that holds an extra bushing in that makes the hole smaller. I took said bushing out, and it opens the hole up to just under a half an inch. Will reinstall and see how the sensor likes it.
 

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2005 Forester X
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@2.5x_sleeper Threw the P2096 today. Maybe I'll try to find a shorter plug non-fouler at the auto parts store. Obviously the sensor needs to see yet more exhaust flow.
 

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2007 Forester Sports XT 4EAT
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@Jfunk if you use the spark plug non-fouler, they come in pairs. One has to be drilled out with a 1/2" drill bit so the O2 sensor tip can extend thru it. Normally, both spark plug non-foulers are used, drilling out only one of the 2.

Bobby...
534200

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2005 Forester LL Bean AT
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Used 1 non-fouler, drilled out, on a mid-05 model. Installed at the end of May, no drive ability issues and the CEL has stayed off so far.
 

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2005 Forester X
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Life got busy, but finally got the chance to track down a set of spark plug non-foulers. Drilled just the one out and reinstalled and unfortunately, no jazz. Got the PO2096 again. Darn! Just tried to drill the hole even bigger than 1/2" to get more flow to the sensor, and still got the PO2096. I guess I can say that I tried...
 

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2005 Forester X
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@2.5x_sleeper The only time I get the PO420 is when I have no spacing installed whatsoever. Anything else I've tried, I get that P2096
 

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2005 Forester LL Bean AT
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Still no problems, CEL out!
After 7 months with no code it threw a P2096, I cleared it and it hasn't come back, (2 weeks), but if the code does return would enlarging the hole in the spacer help?

Thanks.
 

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The Modfather
2019 Impreza 5dr Sport - Manual
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8,070 Posts
It may help. Hard today. My kids 2001 Forester with 210K miles has the fix installed. The CEL will come back every once in a while. Mostly when he takes a long trip like driving home from college. it's a 3+ hr drive. Honestly, I bought a 25 buck code reader and taught him how to check to be sure it's the 440 code then erase it.
 

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2000 4Runner
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406 Posts
So it seems this $5 spark plug non-fouler has been a temporary fix, but more of a permanent solution for many here.. as long as other mechanical issues don’t show up after installing the spark plug non-fouler..
But, has anybody ever gotten to the root cause of why our cars have this dreaded P0420 code?
I’ve read this entire thread and the opinion and suggestions of many, but it doesn’t seem anyone has found out a conclusive answer as to what’s really going on..
I’ve watched a few videos on the topic trying to understand it more..
I saw this mentioned somewhere else and wonder too if this temporary fix might eventually be sending mixed signals to the upstream O2 sensor up front which as we all know is the crucial one for the proper operation of the engine..
I’ll be adding the $5 fix to my car see how it goes.. Thank you all for your knowledge and suggestions.
 

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The Modfather
2019 Impreza 5dr Sport - Manual
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8,070 Posts
My understanding for the rear sensor, on the older foresters at least, like my kids 2001, the rear sensor is strictly to monitor the condition of the cat. converters. Not used for anything else like the AF ratio like the one before the cats.

The fix has been running for quite a while now in the kids Forester. It's been running well otherwise. It seems the light needed to be turned off more frequently during the really cold weather. Really cold being -20°F / -28.9°C which was a cold snap they had in Nebraska last month for an extended period of time. Just something my kid mentioned to me.

I do know from experience that if you have an exhaust leak in the system this can turn on the same code too.
 

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2007 Forester Sports XT 4EAT
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@SubiAficionado prior to '05+, the rear O2 sensor was only used to monitor the efficiency of the CAT. On the '05+, the rear O2 sensor is also used for fuel trim, so the $5 fix is not recommended, as it can cause a P2096 CEL code - Post Catalyst Fuel Trim System Too Lean Bank 1.

Yes, the $5 fix is a workaround, not a fix. Lots of things can cause the excess O2 in the exhaust flow, but most member, with no engine performance issues or reduced gas mileage opt for the $5 & call it good.

We've had the $5 fix on my son's '03 X since the AVO Turbo kit was installed. On full boost, the 5th injector... throttle body injector is programed to kick in, causing an overly rich condition which can cause a P0420 CEL code. The $5 fix prevents that annoyance!

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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2000 4Runner
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Hmm, I see.. In that case I'm glad I have an '01 Forester then. I honestly haven't checked yet for an exhaust leak, hopefully I don't have one. I'll see if the code goes away after the $5 fix.
 

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2007 Forester Sports XT 4EAT
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@SubiAficionado keep in mind the $5 fix can mask problems that should be fixed. Make sure you don't have issues that need to be addressed... like poor engine performance or overly low gas mileage, before installing this workaround.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 
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